1. Field of the Invention
The present invention relates to a frequency modulated signal processing apparatus for use in frequency demodulation for frequency modulating, recording and reproducing an image signal and so on. The present invention particularly relates to such an apparatus which easily and efficiently compensates for deterioration of a frequency modulated signal due to a frequency characteristic of electromagnetic conversion.
2. Description of the Prior Art
An image signal recording/reproducing apparatus records on a magnetic tape a frequency modulated wave which is frequency modulated with an image signal and reproduces the recorded frequency modulated wave. Such an apparatus is encountered with what is generally referred to as "inversion" (described later) during recording of a frequency modulated wave with a high modulation index.
FIG. 1 is a circuitry block diagram of a reproduction system of an image signal recording/reproducing apparatus. In the reproduction system, a frequency modulated signal is reproduced from a magnetic tape 120 by video head 121. The reproduced weak frequency modulated signal is subjected in sequence to amplification at an amplifier 122, correction of its frequency characteristic at an RF equalizer circuit 123, amplitude limiting at a limiting circuit 124, and frequency demodulation at a frequency demodulator 125, before finally reproduced into an image signal by a de-emphasis circuit 126. The image signal recording/reproducing apparatus is haunted by a problem called inversion. Assume that a picture screen has a white region WH and a black region BL as shown in FIG. 2D, for example. The screen is represented by an image signal having a waveform as that shown in FIG. 2B, however, that has undergone frequency demodulation has a waveform shifting from a black level BL to a white level WII as that shown in FIG. 2A, causing that the white region WH partially turns into black as shown in FIG. 2C. The reason is as follows. The image signal recording/reproducing apparatus, due to electromagnetic conversion characteristics, emphasizes a lower sideband of a frequency modulated wave compared with an upper sideband. Zero cross points are eliminated if this happens, depriving the limiting circuit 124 of FIG. 1 of its capability of outputting a normal frequency modulated wave. Frequency demodulation of the abnormal output from the limiting circuit 124 in turn gives rise to an abnormal image signal which has an abrupt level drop as shown in FIG. 2A. In short, the disappearance of the zero cross points invites inversion.
An answer to this problem is to use the circuit of FIG. 3 as the RF equalizer 123 and the circuit of FIG. 4 as the LIM part 124.
FIGS. 3 and 4 respectively show a conventional cosine equalizer and a conventional LIM part. In FIG. 3, indicated at numerical references 1 and 2 are delay lines, indicated at 127 is a first adder, and indicated at 128 is an amplitude adjustor. A phase invertor is indicated at 129 and a second adder is indicated at 130. These are the elements forming the cosine equalizer. More particularly, one end of the delay line 1 is connected to an input terminal and the other end of the delay line 1 is connected to one end of the delay line 2. Two input terminals of the adder 127 are connected to the other end of the delay line 2 and the one end of the delay line 1. An output terminal of the adder 127 is connected to an input terminal of the amplitude adjustor 128 which has its output terminal connected to an input terminal of the phase invertor 129. Two input terminals of the adder 130 are connected to the other terminal of the delay line 1 and an output terminal of the phase invertor 129. An output signal from the adder 130 is an output signal of the cosine equalizer.
In FIG. 4, a delay line is indicated at 131 and a high pass filter (hereinafter "HPF") or a band pass filter (hereinafter "BPF") which represses lower sideband components is indicated at 132. Indicated at 8 and 9 are sign-judging circuits an output from which is H level in response to an input signal exceeding AC 0 volt but L level in response to an input signal smaller than AC 0 volt. An exclusive OR circuit is indicated at 10, a delay line is indicated at 12, an AND circuit is indicated at 13, a sign control circuit is indicated at 14, another delay line for matching propagation delays of signals is indicated at 11, and an adder is indicated at 15. The LIM part is comprised of these elements. More precisely, an output of the cosine equalizer is coupled to one ends of the delay lines 11 and 131 and an input terminal of the HPF 132. The other end of the delay line 131 is connected to an input terminal of the sign-judging circuit 9. An output terminal of the HPF 132 is connected to an input terminal of the sign-judging circuit 8. An output terminal of the sign-judging circuit 8 is connected to one input terminal of the exclusive OR circuit 10. An output terminal of the sign-judging circuit 9 is connected to the other input terminal of the exclusive OR circuit 10 and a reference signal input terminal of the sign-controlling circuit 14. An output terminal of the exclusive OR circuit 10 is connected to one end of the delay line 12 and one input terminal of the AND circuit 13. The other end of other delay line 12 is connected to the another input terminal of the AND circuit 13. An output terminal of the AND circuit 13 is connected to an input terminal of the sign control circuit 14. Two input terminals of the adder 15 are respectively connected to the other end of the delay line 11 and an output terminal of the sign control circuit 14.
Next, operations will be described, first, about the cosine equalizer, and second, about the LIM part. An input signal S.sub.IN given to the input terminal is expressed as: EQU S.sub.IN =E.multidot.e.sup.j.psi.t (1)
Assuming that the delay line 1 implements a time delay of .tau., signals S.sub.A, S.sub.B and S.sub.C at junctions A, B and C, respectively, are given as: EQU S.sub.A =E.multidot.e.sup.j.psi.t EQU S.sub.B =E.multidot.e.sup.j.psi.(t-.tau.) ( 2) EQU S.sub.C =E.multidot.e.sup.j.psi.(t-2.tau.)
From Eq. 2, a signal S.sub.D at a junction D is: EQU S.sub.D =wE.multidot.e.sup..psi.(t-.tau.) .multidot.cos.psi..tau.(3)
The signal S.sub.D is amplified by k times at the amplitude adjustor 128 and thereafter reversed at the phase invertor 129. Hence, a signal S.sub.E at a junction E is given as: EQU S.sub.E =-2.multidot.k.multidot.E.multidot.e.sup.j.psi.(t-.tau.) .multidot.cos.psi..tau. (4)
An output signal S.sub.OUT available at the output terminal of the adder 15 is given as: EQU S.sub.OUT =S.sub.B +S.sub.D =E(1-2.multidot.k.multidot.cos.psi..tau.)E.sup..psi.(t-.tau.) (5)
Eq. 5 states that the output signal S.sub.OUT has a time delay of .tau. compared with the input signal S.sub.IN but shows no phase distortion at all from the input signal S.sub.IN. The amplitude frequency characteristic as shown in Eq. 5 is depicted in FIG. 5A. According to the characteristic of the cosine equalizer as represented in Eq. 5 and FIG. 5A, the amplitude is maximum when .psi..tau.=a.pi.where a is a positive odd value, i.e., when the frequency is f=1/(2a.tau.) (where .psi.=2.pi.f).
The cosine equalizer is used in, for instance, magnetic recording in which a lower sideband is reproduced with a larger amplitude than an upper sideband as shown in FIG. 5B. If the signal processing by the cosine equalizer as set to have an adjusted k is implemented on a signal which is asymmetrical with respect to the frequency of a carrier signal, a resultant signal has a corrected symmetrical spectrum with respect to the frequency of the carrier signal and shows no phase distortion.
Next, the LIM part will be described with the waveform diagrams of FIGS. 6A to 61. FIG. 4 is a circuit receiving a signal of FIG. 6A. The delay line 131 implements as long time delay as a time delay implemented by the HPF 132. If an output from the delay line 131 is at a higher level than the center of its amplitude (AC 0 Volt), an output from the sign-judging circuit 9 has an H level, for example, and if an output from the delay line 131 is at a lower level than the center of the amplitude, an output from the sign-judging circuit 9 has an L level. A signal thus developed has a waveform as that shown in FIG. 6C. On the other hand, the HPF 132 develops a waveform in which zero cross points, if somewhat dislocated from the original zero cross points, are preserved. This signal is fed from the HPF 132 to the sign-judging circuit 8 from which it is outputted as a signal having a waveform of FIG. 6D. The signals of FIGS. 6C and 6D are passed to the exclusive OR circuit 10 and processed therein by computation, yielding a signal as shown in FIG. 6E, i.e., a frequency modulated wave which surges to the H level only at the disappearance or the deviation of the zero cross points. By extracting a pulse which remains at the H level for a relatively long period of time, precisely, more than a time t, and thereafter processing the consequent signal in light of the polarity of the original frequency modulated wave, the zero cross points of the original frequency modulated wave are restored. The time t is given as follows. ##EQU1##
Hence, by delaying the waveform of FIG. 6E and thereby generating a waveform of FIG. 6F, and thereafter processing the signals of FIGS. 6E and 6F in the AND circuit 13 in such a manner that a pulse which corresponds to the disappearance of the zero cross points is extracted, a signal of FIG. 6G is obtained. Superimposition of the extracted pulse requires consideration on the polarity. That is, in order to restore the zero cross points in the output signal of FIG. 6C from the cosine equalizer, the pulse as it is reversed to go negative is superimposed on the input frequency modulated wave since the disappearance of the zero cross points is observed while the signal of FIG. 6C stays at the H level. On the contrary, if the disappearance of the zero cross points is observed while the signal of FIG. 6C stays at the L level, the extracted positive going pulse as it is superimposed on the input frequency modulated wave. This process is accomplished in the sign control circuit 14. Hence, the sign control circuit 14 converts the signal of FIG. 6G into a signal as that shown in FIG. 6H which will be superimposed on the frequency modulated wave from the cosine equalizer in the adder 15, whereby a signal as that shown in FIG. 6I is generated. As shown in FIG. 6I, the zero cross points are restored in the resulting waveform. Moreover, since superimposition of the upper sideband components having a poor C-N ratio is executed only where the zero cross points were erased, demodulation with an excellent S-N ratio is attained without causing post-demodulation changes in frequency characteristic. In short, change in inversion suppression effect would not lead to variation or distortion in frequency characteristic of a demodulated image signal.
Thus, in the conventional apparatus for processing frequency modulated signals, the RF equalizer 123 performs waveform correction without causing phase distortion, and the LIM part 124 restores only disappeared zero cross points, which advantageously attains no deterioration in an S-N ratio, correction of frequency characteristic and suppression of the inversion. However, its design complexity due to a number of components required therein cannot be blind-eyed.